The present invention relates to mass spectrometers, which are important in analytical field, in particular, to plasma ion source mass spectrometers and liquid chromatography/mass spectrometers.
A plasma ion source mass spectrometer, wherein a sample is introduced into plasma at a high temperature and ionized by heat of the plasma, has been used in the field of chemical element analysis. Any of inductively coupled plasma and microwave plasma can be utilized for the ion source.
A conventional plasma ion source mass spectrometer is explained briefly referring to FIG. 21, hereinafter.
A liquid sample in a sample bottle 1 is nebulized in a spray chamber 2, and ionized by introducing into plasma 3. The generated ions are introduced into a differential pumping region 34 (approximately one Torr), which is pumped by a rotary pump 124, via a sampling cone 4. The ions entered the differential pumping region 34 are introduced into a vacuum region 35 (lower than approximately 10xe2x88x924 Torr.), which is pumped by a turbo molecule pump 125, via a skimmer cone 5. Orbits of the ions passed through a gate valve 8 are collimated by ion lenses 22a, 22b, 22c, and an entrance aperture 19 in the vacuum region 35. Subsequently, the ions are deflected by 90 degrees using a deflector composed of quarterly split cylindrical electrodes 10a, 10b, 10c, and 10d (hereinafter, this deflector is called Q-deflector) for separating the ion from photon, which is a main source of noises. Then, collimation of the ions is improved by correcting electrodes 16a and 16b, and the ions are introduced into the mass analyzer 14 for performing mass separation, and detected by a detector 15.
In accordance with using the Q-deflector as explained above, the photons and neutral molecules generated by the plasma are prevented from reaching the detector, and mass spectrum having a preferable ratio of signals versus noises (hereinafter, it is called S/N ratio) can be obtained.
In order to obtain a stable intensity of ions, the pressure in the vacuum region, wherein the mass analyzer is arranged, must be stabilized. Because it takes approximately one hour for stabilizing the pressure in the pressure chamber of the vacuum region 35 from start of pumping, the turbo molecule pump 125 for pumping the vacuum region 35 is preferably not stopped.
On the other hand, the pressure in the differential pumping region 34 can be stabilized in approximately one minute, because its reached pressure of pumping is relatively high in comparison with the pressure of the vacuum region. Therefore, a leading electrode 23 provided with an aperture having a shielding function to prevent neutral particles from entering is arranged between the gate valve 8 and the skimmer cone 5. In order to restrict a portion to be contaminated in the vacuum region, the major portion of the neutral particles generated in the ion source are adhered to the leading electrode 23. If the electrode is contaminated, an insulating film is formed and a charging phenomenon is occurred, and an electric field for transmitting the ions effectively is disturbed. Maintenance of the apparatus can be performed with pumping the vacuum region continuously by shutting the gate valve and cleaning the leading electrode 23. Therefore, the time necessary for re-start up of the apparatus can be shortened.
The conventional plasma ion source mass spectrometer described above was disclosed in JP-A-7-78590 (1995) and JP-A-9-306418 (1997).
Currently, an apparatus, wherein a plasma ion source and a quadrupole ion trap mass spectrometer (hereinafter, called ion trap) are combined, has been used. The apparatus disclosed in xe2x80x9cRAPID COMMUNICATIONS IN MASS SPECTROMETRYxe2x80x9d vol. 8, 71-76 (1994) is explained hereinafter referring to FIG. 22.
Ions generated by plasma are led to the first differential pumping region 36, which is being pumped by a rotary pump 124, and subsequently, the ions are introduced into the second differential pumping region 37 by a leading electrode 26. After passing through the gate valve 8, the ion orbits, which have been scattered by a photon shielding electrode 27 called as a photon stopper, are collimated by ion lenses (28a, 28b), and introduced into the vacuum region 35. The ions are collimated again with the ion lenses (29a, 29b, 29c), and introduced into the quadrupole mass spectrometer 30. In accordance with the quadrupole mass analyzer 30, ions having a specified mass number can be transmitted selectively. Subsequently, the ions are collimated with the ion lenses (31a, 31b, 31c), and introduced into the ion trap mass analyzer 14. The ions are detected by the detector 15 after being separated depending on their masses.
In the field of organic substance analysis, a liquid chromatograph/mass spectrometer (hereinafter, called LC/MS), which is composed by combining a liquid chromatograph and a mass spectrometer, has been used frequently. In accordance with the LC/MS, ions are separated from neutral particles such as micro-droplets and the like for increasing a S/N ratio. A conventional example of the LC/MS has been disclosed in U.S. Pat. No. 5,481,107.
In accordance with the conventional plasma ion source mass spectrometer, noises can be decreased, because entering the neutral particles is prevented by reducing a part of the leading electrode 23 for decreasing a sighting angle, or using the photon stopper 27 on the axis of the skimmer cone. On the other hand, a problem to decrease ion transmission is generated. In some cases, reproducibility of the ion transmission can not be obtained, because of errors in assembling the lenses after cleaning.
Generally, the gate valve 8 is electrically grounded. Therefore, the ions move slowly at this region, and the ions are readily effected by unintentional seeping of electrical field (called fringing field) from the electrode arranged in the vicinity of the gate valve. Generally, the unintentional electrical field is formed asymmetrically to the central axis of the lens. Accordingly, the ion lenses 22a, 22b, 22c, which are axially symmetrical, and an entrance aperture 19 have a problem to decrease the ion transmission by collimating the ion orbit insufficiently.
Furthermore, the conventional plasma ion source mass spectrometer indicated in FIG. 21 was composed of two pairs of flat plates, each of the pairs faced each other (16a, 16b, and a pair of plates arranged at above and beneath this paper, which are not indicated in the figure), as correcting lenses for introducing the ions into the mass analyzer after the ions were deflected. That is, a moving direction of the ions is corrected and collimated by giving a potential gradient in a direction perpendicular to the moving direction of the ions deflected by the deflector. Therefore, because the most optimum voltage of the correcting electrodes 16a, 16b depends strongly on kinetic energy of the ions, collimating the ions is difficult with the plasma ion source mass spectrometer, which has a large fluctuation in the kinetic energy of the ions.
That is, it is an issue that increasing the efficiency of the ion transportation by how eliminating particles other than the ions during introducing the ions from the ion source to the mass spectrometer for analyzing the ions. Therefore, the ion transport region for introducing the ions into the ion mass spectrometer has a fundamental composition, wherein the ions are introduced into the analyzing region after the ion orbit is remarkably bent in the ion transport region.
In some cases, reproducibility of the ion transmission can not be obtained, because of errors in assembling the lenses after cleaning.
In accordance with the conventional mass spectrometer, it is a problem that molecule ions such as ArOH+ and NOH+ are generated from argon gas or nitrogen gas used as a carrier gas for sample or plasma gas, and these molecule ions disturb the measurement by overlapping with peaks of atomic ions of the measuring object.
In accordance with the ion trap mass analyzer, it is a problem that an error in the measurement is generated by decreasing the number of the measuring object ions by reacting the ions in the mass analyzer with impurities caused by vacuum pump oil and the like during storing the ions for several to hundreds ms before the analysis.
The first object of the present invention is to provide a mass spectrometer having a high S/N ratio by a high ion transmission and effective elimination of neutral components.
The second object of the present invention is to improve accuracy in assembling the ion lenses.
The third object of the present invention is to provide a mass spectrometer, wherein effects of molecule ions are reduced.
The fourth object of the present invention is to improve an accuracy in measurement by controlling undesired reactions in the mass spectrometer.
In accordance with the present invention, the mass spectrometer comprising: an ion source arranged in a first chamber under a first pressure condition; an orifice for introducing ions generated in the ion source into a second chamber under a second pressure condition, which is lower than the first pressure condition; an ion transport region for transporting the ions introduced into the second chamber through the orifice to a desired region; and a mass analyzer for analyzing the ions transported through the ion transport region by mass spectrometry; wherein a gate valve is provided between electrodes composing the ion transport region, a shielding plate having a small opening is provided between the orifice and the gate valve, and a leading electrode for collimating the ions is provided between the orifice and the shielding plate; is used for solving the above problems.
In order to increase the accuracy in assembling the leading electrode and the shielding plate, the leading electrode and the shielding plate are fixed on a flat plate having plural openings. A differential pumping region under a third pressure condition, which is intermediate between the first pressure condition and the second pressure condition, is provided between the first chamber and the second chamber. For the above ion source, any one of plasma ion source, electrospray ion source, atmospheric chemical ionization ion source, and sonicspray ion source can be used. The ion trap mass analyzer can be used as the above mass analyzer. The ion transport region can be composed of ion collimating lenses and a deflector. A double cylindrical static lens can be used for any one of the ion collimating lenses. In accordance with the deflector, the ion orbit can be deflected by approximately 90 degrees. The deflector can be composed of plural sector-shaped electrodes. The deflector may be a quadrupole deflector composed of quarterly split cylinder-shaped electrodes. In order to eliminate molecule ions, a power source for applying high frequency waves of desired frequencies is provided between a pair of end-cap electrodes composing the ion trap mass analyzer. The distance between the end-cap electrodes can be set longer than 20 mm. The end-cap electrode comprises an opening, and the diameter of the opening can be set larger than 2 mm. An oil-free scroll pump can be used for pumping the differential pumping region.
In accordance with the mass spectrometer of the present invention, the ions of the measuring object are detected by introducing into the mass analyzer through the leading electrodes and ion lenses after passing through the skimmer cone. The photons and neutral molecules generated by the plasma are adhered to the shielding plate having a small opening at a portion out of the moving direction of the ions. Because the major portion of the neutral molecules are adhered to the shielding plate, the contamination can be prevented at the mass analyzer side from the gate valve side. In accordance with suppressing the contamination, the disturbance of the electric field can be prevented, and the ion transmission can be readily maintained high. Because the shielding plate is located before the gate valve, maintenance such as cleaning and the like can be performed without stopping operation of the vacuum pump for the vacuum region. Therefore, re-measurement after cleaning can be performed easily. Furthermore, the neutral molecules and photons passing through the shielding plate can be separated from the ion orbits by deflecting the ions approximately 90 degrees by the deflector. In order to maintain the high ion transmission with such ionic optical system as described above, ion lens (static lens) composition and its assembling method were improved, and the leading electrodes and the shielding plate were fixed onto the flat plat having plural openings. Accordingly, the errors in assembling the ionic optical system could be decreased, and the reproducibility of the ion transmission could be improved. In accordance with the present invention, a function to control the kinetic energy of the ions introduced into the mass analyzer could be realized, and significant improvement in the S/N ratio was realized by remarkable improvement in the efficiency of ion accumulation into the ion trap mass analyzer.
The measurement with a high sensitivity became possible by separating the atomic ions of the measuring object from matrix molecule ions by using ion trap mass analyzer for the mass analyzer of the plasma ion source mass spectrometer, and performing operations such as applying a resonance voltage between the end-cap electrodes, and others.